Traveling wave tubes



May 1, 1-962 .E. C. DENCH TRAVELING WAVE TUBES Filed May 8, 1959 lNVE/VTOR EDWARD 0. DE/VGH /K MM A TTORNE Y 3,032,677 TRAVELING WAVE TUBES Edward C. Dench, Needham, Mass, assignor to Raytheon Company, Waltham, Mass., a corporation of Delaware Filed May 8, 1959, Ser. No. 812,967 4 Claims. (Cl. 315-35) This invention relates to a crossed-field traveling wave tube, and, more specifically, to a grooved collector for a crossed-field-type traveling Wave tube which permits collection of the electron beam over an extended collector area, thereby providing for adequate cooling of the collector.

Traveling wave tubes are known which include a slow wave energy propagating structure or delay line, a continuous electrode or sole spaced from and disposed substantially parallel to said structure, and an electron gun mounted adjacent one end of said structure for producing an electron beam which, under the influence of an electric field existing between said structure and said sole and a magnetic field transverse to the electric field and to the beam, traverses the interaction space bounded by the slow wave structure and the sole. Such traveling wave tubes are widely used either as amplifiers capable of operation over a large band Width or as oscillators capable of being tuned electronically over a considerable frequency range in the microwave region. Such devices utilize the interaction between an electron beam moving along paths adjacent a periodic nonresonant slow wave propagating structure and the electromagnetic field of the radio frequency wave propagating along said periodic structure. The electromagnetic field along such a periodic structure may be resolved into a number of superimposed traveling waves or space harmonies each having its own phase velocity. Some of the space harmonics of a phase velocity travel in the same direction as the wave energy or group velocity and are referred to as forward waves. Other space harmonies, on the other hand, have a phase velocity of opposite sense, that is, the phase velocity is in a direction opposite to the energy or group velocity. Such harmonies are referred to as backward waves. If the electron beam velocity is adjusted, as by varying the ratio of the intensity of the electric field between the delay line and the sole to the strength of the transverse magnetic field, so that it is in substantial synchronism with the phase velocity of a given space harmonic, interaction between the electron beam and this component will occur, and energy will be transferred from the electron beam to the electromagnetic field. In a traveling wave amplifier, a radio frequency input signal is coupled to the periodic structure adjacent one end thereof, and, owing to the interaction between electron beams moving along a path adjacent the periodic structure and the electromagnetic field of the radio frequency wave propagating along said structure, amplification of the input signal may be obtained, under proper operating conditions. This amplified signal may then be extracted from the periodic structure adjacent the other (output) end thereof. In the forward wave amplifier the interaction is between the electron beam and a forward wave; the electrons thus are projected toward the output end of the periodic structure. In the backward wave amplifier, interaction occurs between the electron beam and a backward wave and the electrons then move toward the input end of the periodic structure. In a traveling wave oscillator, when the electron beam current exceeds a critical current at which oscillations can begin, and when the electron beam velocity is substantially equal to the velocity of a suitable space harmonic of the propagated wave, oscillations may be generated within the device and the States ldatent ice generated energy will propagate along the periodic structure and may be extracted at one end thereof.

As the power rating of such traveling wave tubes increases, the problem of conducting sufficient heat energy from the tube to prevent its overheating or destruction becomes more difficult.

In the crossed field tube already referred to there are two predominant forces acting upon the electron, namely, the force urging the electrons toward the anode or delay line and the force urging the electron toward the sole. The first-mentioned force is equal to the product of the strength E of the electric field in which the electron is located and the charge e on the electron. The secondmentioned force is equal to the product of the strength 13 of the magnetic field in which the electron is positioned, the charge e on the electron, and the velocity v of the electron. When these two forces on the electron are equal, taking into account the centrifugal force in a curved system, the electron beam travels either in a straight line or along a curved path. As energy is transferred from the electron beam to the high frequency field along the delay line, the electron beam velocity tends to decrease and the force urging the electrons toward the delay line becomes predominant. Consequently, the electrons move toward the delay line and may be collected by a collector electrode positioned at or near the downstream end of the delay line. In tubes using collectors of conventional design, however, the beam tends to be concentrated over a relatively small collector area, resulting in erosion, or even melting, of the collector electrode. Even when fluid cooling is provided for the delay line, the concentration of the electron beam over a relatively small area will be such that fluid cannot carry away heat at a sufficiently rapid rate to avoid overheating or burnout. Much of the difficulty in heat transfer through fluid cooling means arises from califaction within the fluid; that is, boiling of the fluid increases the thermal resistance of the fluid coolant considerably.

In accordance with the invention, the electron beam is collected gradually over an extended collector area by causing the electron beam to impinge upon a collector whose surface facing the electron beam is provided with longitudinal grooves along the direction of electron beam travel. Electrons entering the grooves in the collector electrode will be shielded from the electric field, that is, the electric field strength will be reduced in the grooves. Consequently, the force Ee tending to urge the electrons toward the delay line will be reduced, and the force Bev tending to urge the electrons toward the sole will predominate. Thus the electrons in the grooves will move toward the sole-collector interaction space. The electrons emanating from the grooves will encounter the normal electric field in the sole-collector interaction space and will again be urged toward the more positive collector electrode. Some of these electrodes, in turn, will pass into the grooves in the collector, and so on, until most of the electrons are finally collected. because of mutual repulsion between electrons, the electrons emanating from the grooves will be redistributed laterally and the beam will tend to fan out across the collector; that is to say, some of the electrons originally entering a given longitudinal groove may, upon again approaching the collector, enter other longitudinal grooves. This process is repeated several times until all, or substantially all, of the electron beam is collected. This total collection will occur over an extended area, thereby permitting adequate cooling of the collector electrode.

If the groove width be assumed equal to the spacing between grooves, approximately half of the electrons in the beam will encounter these solid projecting areas of the In addition,

collector electrode and substantially half will enter the grooves. Inside the grooves, the force Bev will predominate, causing these electrons to move toward the sole and re-enter the sole-collector interaction space. Space charge forces then redistribute the electrons in this half of the electrons; about half of these redistributed electrons will, in turn, be intercepted and the remaining half (about 25 percent of the entire electron beam) made to re-enter the sole-collector space, thereby leaving about a quarter of the electrons to be subjected to the forces in the solecollector space, etc., until substantially all of the electrons have been collected. The proportions of the collecting area (projections) to the groove area are not limited to the 50-50 ratio above-mentioned; this ratio is illustrative only. The ratios will vary from tube to tube and will be such as to optimize collection for the given tube.

Other objects and features of this invention will be understood more clearly and fully from the following detailed description of the invention with reference to the accompanying drawing wherein:

FIG. 1 is a cross-sectional view, partly in elevation, of a traveling wave tube showing a collector electrode according to the invention;

FIG. 2 is a cross-sectional view, partly in elevation, of the traveling wave tube of FIG. 1 taken along the line 2-2;

FIG. 3 is a pictorial view showing details of the collector electrode shown in FIG. 2;

FIG. 4 is a detail view showing a collector electrode which is maintained at a potential different from that of the cylindrical outer portion of the delay structure;

PEG. 5 is a cross-sectional View of the traveling wave tube of PEG. 1 having a modified collector electrode arrangement; and

FIG. 6 is a detail view, taken along the line 66 of FIG. 5'.

Referring to the drawing, a traveling wave tube 16' is shown which comprises a slow wave energy-propagating structure or delay line 12, a cylindrical electrode 14, otherwise referred to as a sole, which is concentric with the delay line 12 and normally maintained negative with respect thereto, a lead-in assembly 15, an electron gun assembly 29 including at least a cathode 21 and heater 22, a magnetic field-producing means 25 and an output coupling means 17. The circular delay line 12 includes several interdigital fingers or elements 31 and 32 extending from respective oppositely disposed annular members 33 and 34. Members 33 and 34. are secured by screws 35, visible in FIG. 1, to the shoulder portion of a cylindrical electrically-conductive ring 35, said ring forming a part of an evacuated envelope for tube It A pair of oppositely located cover plates 38 and 39 hermetically sealed to ring 36 also form a portion of the tube envelope. Although the delay line has been described and illustrated as an interdigital network structure, the invention is not limited thereto; the delay line may consist, for example, of a strapped vane structure, a loop-loaded wave guide, a strap: ped loop structure, a disk-loaded coaxial structure, or any type of ladder line.

The sole 14 consists essentially of a cylindrical block constructed of an electrically-conductive material and including a web portion 14a bounded by an arcuate section 14b whose periphery consists of an active surface and side or edge members 14c. The purpose of these side members 14c is to confine the electron beam within the interaction space 48 between surface 45 of sole 14 and the interdigital line 12. One end of a hollow supporting member 52 is inserted within a central aperture in sole 14 and is secured firmly to the sole. Supporting member 52, in addition to providing support for sole 14, forms a portion of lead-in assembly 15 and allows for passage of external circuit connecting leads, in a manner to be described subsequently.

The sole 14 contains a slot 519 for accommodating the electron gun assembly 29. Since the invention does not involve details of the electron gun 20, the latter is shown only in FIGS. 2 and 5 and, even then, only schematically. The construction and manner of mounting of the electron gun 20 may be as show in a copending application of Roy A. Paananen, Serial No. 717,897, filed February 27, 1958, now Patent No. 2,914,700. Electron gun 20 includes a cathode 21, a heater 22, a grid electrode 23 which may be used for control of beam current (as for amplitude modulation), and an accelerating electrode 24 which likewise may be used for control of beam current. The

cathode 21 may be in the form of a rectangular prism.

provided with a circular bore in which a heater wire 22 is inserted and electrically insulated from cathode 21; the heater may be connected at one end to the cathode. Electrical energy from appropriate sources is supplied to the cathode 21, heater 22, grid electrode 23 and accelerating electrode 24 by way of respective lead-in wires 61, 62, 63, and 64, which are brought out from the tube envelope through the lead-in assembly 15. In FIG. 1, the electron gun 2t) has been omitted, for the sake of simplicity and clarity and in order not to obscure other details of the tube.

Lead-in assembly 15 includes an electrically-conductive sleeve 66 affixed to the inner periphery of cover plate 38, as indicated in FIG. 1. A section of cylindrical glass tubing 67 interconnects sleeve 66 and a second electricallyconductive sleeve 68. The other end of tube 68 is provided with a glass seal 69 for sealing the traveling wave tube 10 after evacuation. One end of Sole-supporting member 52 contains an outwardly flared portion which is connected to the inner surface of sleeve 68. The leads 61, 62, 63, and '64 are mounted in electrically-insulated rela-t tion with supporting member 52 by one or more glass beads 71.

The coaxial output coupling means 17 is sealed in an opening of wall 36 of delay line 12 and is impedancematched to the delay line. The inner conductor 72 of the output coupling means 17 is connected to a finger of delay line 12 at or near the end of the delay line adjacent electron gun 2! The necessary electric field between the slow-wave structure 12 and sole 14 may be obtained by means of a unidirectional voltage applied therebetween; such a voltage may be supplied by a battery 33. The sole 14 may be biased negatively relative to the cathode 21 by means of a source 81 of voltage connected between cathode lead 61 and sole-supporting member 52 by Way of sleeve 68. The cathode 21 may in some instances, however, be at the same potential as sole 14; in this case, source 81 would be omitted. Similarly, the delay line 12 may be maintained at a potential positive relative to both sole 14 and cathode 21 by means of the source 83 of unidirectional voltage connected between the cathode and Sleeve 66, the latter being connected, in turn, to delay line 12. The accelerating electrode 24 may be maintained at a potential positive relative to cathode 21 by means of a source 85 of unidirectional voltage connected between leads 61 and 64-. The control grid lead 63 may, if desired, be connected by way of terminal 38 to an appropriate energy source for controlling the magnitude of the electron beam current in the traveling wave oscillator 19. A suitable heater voltage from a source 32 is connected between heater lead 62 and cathode lead 61.

A uniform magnetic field transverse to the direction of propagation of the electron beam is provided by a permaent magnet or electromagnet having cylindrical pole ieces 91 and 92 radially positioned on or adjacent the anode cover plates 33 and 39, respectively. Pole piece 91 is apertured to receive lead-in assembly 15, while pole piece 92 is apertured to maintain symmetry of the magnetic field. The flux lines should be concentrated in the interaction space 48 between sole 14 and delay line 12. By a proper adjustment of the magnitude and polarity of the magnetic and electric fields so established, the electron beam may be made to move along a more or less circular path along the interaction space 48 under the combined influence of these transversely disposed fields.

Although the tube which has been described may be used as an oscillator, the invention also is applicable to a traveling wave amplifier, which structurally may be the same as the oscillator 10 of FIGS. 1 and 2 except that an additional energy coupling means would be provided at the end of the delay structure 12 opposite the output coupling means 17'.

The traveling wave tube 10 is provided with a collector electrode 75 which contains longitudinally arranged grooves 77, that is, grooves arranged in the direction of the electron beam. The collector may be secured, as by Welding, directly to the inner surface 37 of the conductive ring 36, as indicated in FIGS. 1 to 3, 5 and 6. The collector electrode 75 is tapered so that its radial dimension is decreased in the direction of movement of the electron beam. The degree of tapering will depend upon a particular tube design, including such factors as power handling requirements, tube operating voltages, etc. The collector electrode of FIGS. 1 to 3, 5 and 6, being in contact with the large thermally-conductive ring 36, is able to dissipate heat quite readily, particularly if the ring 36 is surrounded by fiuid-cooling means. In order to insure total collection of the electron beam, a post collector 76 may be provided. This collector, which is not always necessary, may project inwardly from the inner peripery 37 of the conductive ring 36 of tube 10. Although not shown in FIGS. 1 to 3 and 5, cooling of the delay line and the collector 75 may be improved by means of a fluid cooling header surrounding the conductive ring 36 and in thermal contact therewith. Such a header is indicated in FIG. 6 by the reference numeral 120.

The collector electrode need not be connected directly to the delay line, but may be electrically insulated from the conductive ring 36, as shown in FIG. 4. The support assembly 95 of FIG. 4 includes a tube 96 sealed to ring 36. A tubular member 97 is attached to tube 96 by way of a glass seal 98. An inlet water pipe 101 and outlet water pipe 102 are brought up through member 97 and are sealed to member 97. The grooved collector electrode 75 of FIG. 4 contains a curvilinear conduit or passage, not shown, into the ends of which the inlet and outlet water pipe 101 and 102 are inserted. The Water pipes may be brazed to the openings in the passage or may be provided with threaded fittings engaging threaded ends of the passage. These water pipes may be made sufficiently rigid to provide mechanical support for collector 75. An appropriate bias voltage between the collector 75 and the delay line 12 may be provided by a battery 105 or other source of unidirectional voltage connected, for example, between the tube 96 and member 97.

In some cases, as shown in FIG. 5, the collector electrode 75 may contain more than one tapered surface. For example, the grooved collector 75 of FIGS. 5 and 6 is doubly tapered. Also, as shown in FIGS. 5 and 6, electron beam collection may be enhanced by providing an additional electrode 115 substantially parallel with the collector electrode 75. This additional electrode 115 may be attached to the inactive portion of sole 15, as by screws 118, and has the surface or surfaces facing the collector electrode 75 disposed substantially parallel to the juxtaposed surface or surfaces of the collector electrode. A fluid-cooling duct or housing 120 surrounding the tube envelope portion 36 may be provided to assist in removal of heat from the collector and from the delay line.

This invention is not limited to the particular details of construction, materials and processes described, as many equivalents will suggest themselves to those skilled in the art. For example, the invention may be applied to tubes of linear configuration, as well as to tubes of cylindrical configuration. It is accordingly desired that the appended claims be given a broad interpretation commensurate with the scope of the invention within the art.

What is claimed is:

1. A traveling Wave tube comprising a periodic slow Wave energy propagating structure, means for emitting and projecting a beam of electrons along a path adjacent said slow wave structure, and electron collecting means including a surface inclined to said path, said surface being provided With grooves extending along said path.

2. A traveling wave tube comprising a periodic slow wave energy propagating structure, means for emitting and projecting a beam of electrons along a path adjacent said slow wave structure, and electron collecting means including a surface inclined to said path, at least a portion of said collecting means being provided with grooves positioned along the direction of travel of the electron beam.

3. A traveling wave tube comprising a periodic slow wave energy propagating structure, means for emitting a beam of electrons, crossed electric and magnetic fields compelling said electrons to travel along a path adjacent said slow wave structure, and electron collecting means including a surface inclined away from said path in the direction of travel of said electron beam, said collector electrode being provided with grooves positioned along the direction of travel of the electron beam.

4. -In combination, a periodic slow wave energy propagating structure constructed to produce along a path adjacent thereto fields of electromagnetic Wave energy being transmitted, means including a cathode and transverse electric and magnetic fields for directing a beam of electrons through mutually perpendicular electric and magnetic fields and along said path in energy-exchanging relation with said wave fields, a sole electrode coextensive with the said structure end, and a collector electrode disposed along said path at the end thereof, the surface of said collector electrode facing said beam being inclined to said path and containing at least one depression extending along the direction of travel of said electron beam and defining a region wherein said electric field is materially reduced.

References Cited in the file of this patent UNITED STATES PATENTS 2,680,209 Veronda June 1, 1954 2,824,996 Rogers et a1 Feb. 25, 1958 2,860,277 Iversen Nov. 11, 1958 2,889,488 Reverdin June 2, 1959 2,890,375 Ruggles June 9, 1959 2,949,558 Kornpfner et a1 Aug. 16, 1960 FOREIGN PATENTS 781,205 Great Britain Aug. 14, l957 

